Clastic Intrusions EarthCache

This cache has been archived.

egroeg: While driving in the area today, I went past this cache to see how it was holding up. Unfortunately, the trees and other growth have flourished so much that you really can't see the rock face anymore.
So, I am reluctantly archiving this EarthCache. It was one of my favorites, and really was a unique location in this area. Thanks to everyone who visited, and I hope you found it as interesting as I did.

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Hidden : 8/18/2015
Difficulty:
2.5 out of 5
Terrain:
1 out of 5

Size: Size: other (other)

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Geocache Description:

This is an EarthCache, so THERE IS NO CONTAINER AT THIS LOCATION. For an EarthCache, you make observations as part of an earth science lesson and report your findings to the cache owner. Failure to send them may result in the deletion of your log. For more information about EarthCaches, visit EarthCache.org

INTRODUCTION


"Solid as a rock." "Rock steady." "Hard as a rock." "Carved in stone." There are many expressions relating to our impression that the rocks that make up our world are a source of strength and permanence. But there is a lot going on under our feet that may make you change your mind.

The rocks in our area can be exposed in road cuts (like the scene above) or anywhere a hillside has been cut away. Locally, these rock layers (or beds) are seen to be regular and horizontal, looking much the way they did when they were deposited millions of years ago. The beds were not rocks at first, they were laid down as layers of sediment, like sand (from rivers, beaches, or deserts) or mud (from lakes, ponds, or mudflats). Only with time, pressure, and heat were the deposits turned to stone. (The process of changing soft sediments to stone is lithification.) But sometimes, under certain conditions, interesting things can happen to the sediment beds before they are lithified.
During the Late Pennsylvanian Period, about 300 million years ago, Allegheny County was the site of a large river delta. The river arose to the south and east, where the Allegheny Mountains were forming as a result of the collision between North America and Africa. This river carried huge amounts of sediments, which were deposited as layers of sand (in the river channels) and mud (on the floodplains beside the river). As the river regularly changed course, these different layers might get stacked upon each other, building up what would become the alternating sandstones and shales we see in the roadcuts. First, though, must come lithification.

Lithification requires the compacting of the sediment layer, which also squeezes out water, and cementing the sediment grains together with minerals from the water. The compacting force can be vertical, from the weight of the layers above, or from below if there is volcanic activity. The force may also be horizontal, from the collision of continents. The actual situation is likely some combination of all these forces.
Compaction and dewatering of different types of sediment beds can occur at different rates. Mud and silt, for example, are made up of extremely fine particles. They will compact easily, and water is quickly expelled. Some of these mud layers may even become lithified while the neighboring sands are still only partially solid. In some extreme cases, a wet sand deposit may be completely enclosed by lithified mud (which we'll call shale), and if the water trying to leave the sand cannot penetrate the hard shale, it remains in the sand. What happens to that wet sand as the pressure continues to grow? In some cases, you get an EarthCache.

A mixture of small insoluble particles (like sand) and a liquid (like water) forms a slurry. A slurry can be poured, like ketchup or concrete, or it can flow under pressure, like toothpaste from a tube. The pressurized sand slurry enclosed by shale has nowhere to flow - until cracks form in the shale. Then, the sand slurry will be injected into the cracks in the shale. If the pressure is high enough, the slurry may cause more cracking and travel through many layers of rocks, until the pressure is reduced and it can't make more cracks. The slurry can travel vertically (either up or down) or horizontally, depending upon where it finds weaknesses in the rocks. The slurry has formed an intrusion into the other rock layer. An intrusion that cuts up or down through a layer is a dike and a horizontal intrusion is a sill. In this photo, a tan sand slurry has intruded into the gray rock, and then the tan material became lithified. When an intruding slurry becomes lithified, it is called an injectite and the rock that has been intruded upon is known as the host rock.

But cracks in rocks are very narrow, so how did that tan injectite get so wide? The slurry is under so much pressure that it can break off bits of the gray sandstone and carry them off when it flows further. You can see in this photo that the gray host rock has been fractured by the slurry. Wait a minute - a sand slurry injected under pressure has fractured the host rock and created a channel? That's fracking!! Nature invented fracking hundreds of millions of years ago!


Intrusions are identified by the type of material that is flowing - a volcanic intrusion is molten rock intruding into other rocks, and a clastic intrusion is when soft sediments, like sand or mud, intrude into other rocks. Now that I've finally explained what the title of this EarthCache means, we can start looking more closely at this location.

EARTHCACHE


The Mt. Nebo Pointe shopping plaza is located just off the Camp Horne RD exit of I-279. Along the wall at the north end of the parking lot, an amazing network of clastic intrusions is exposed. FOR YOUR SAFETY, DO NOT APPROACH THE ROCK WALL - all requirements of this EarthCache can be met without leaving the parking lot. No special tools are required.



SITE 1:

QUESTION 1: Take a moment to look at the tan and gray rocks. Based on your first impressions, Was the tan injectite forced DOWN into the gray sandstone host rock, or was it forced UP into the gray sandstone? (There is no right or wrong answer here, I just want your opinion.)

At the locations labeled A1 and A2 you can see two sets of fractured host rock.

QUESTION 2: A small diagonal sill begins at A2. Follow this sill to location B, take a close look, and tell me What is different about this sill at B?

QUESTION 3: To give you an idea of how much material might have been injected, Estimate the thickness of the diagonal dike at location C.

SITE 2:

Walk a short distance to the right to Site 2. There are three dikes between here and the light pole that is further to the right.
QUESTION 4: What are the differences in size and spacing of the dikes here, compared to Site 1?(Thicker, thinner, closer, further, etc.)
QUESTION 5: There are several sills here. Are they in the top or bottom half of the gray sandstone layer? Look only at the gray rock, not the layers above and below it.

SITE 3:

After another short walk to your right, there will be three more dikes, starting at this spot and going further to the right. No questions about the dikes or sills here, just a chance to see how widespread the intrusions are. There is one question, though...
QUESTION 6: Estimate the total height of the gray host rock exposed here. This should give you an idea of the tremendous pressures needed to force a slurry that far.

QUESTION 7: Based on all you've seen, what do you now think: Was the tan injectite forced DOWN into the gray sandstone or was it forced UP into the host rock?

Please remember to send your answers to me when you log this cache. If you post any photos in your log, try not to give away any of the answers.


Reference: lithification diagram from http://www.slideshare.net/mohammedalmusawi666666/ch07-sedimentary-rocks

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